Studying the effects of aerosols on vertical photolysis rate coefficient and temperature profiles over an urban airshed

This paper discusses the effects of size‐ and composition‐resolved aerosols on photolysis and temperatures within and above an urban airshed. With respect to photolysis, three‐dimensional simulations indicated that (1) in regions of the boundary layer where absorption of ultraviolet (UV) radiation was strong, aerosols reduced photolysis coefficients of UV‐absorbing gases; (2) in regions of the boundary layer where UV scattering dominated UV absorption by aerosols, aerosols enhanced photolysis coefficients of UV‐absorbing gases; (3) aerosols increased photolysis coefficients for visible‐absorbing gases since visible scattering always exceeded visible absorption by aerosols; (4) scattering and weakly absorbing aerosols above the boundary layer increased photolysis coefficients above the boundary layer for all absorbing gases; and (5) increases in aerosol absorption extinction within the boundary layer reduced photolysis coefficients above the boundary layer for all absorbing gases. Photolysis coefficients changes due to aerosols decreased near‐surface ozone mixing ratios in Los Angeles by 5–8%. With respect to temperatures, simulations indicated that aerosols increased radiative heating rates at all altitudes but decreased surface solar irradiances during the day. Surface irradiance reductions cooled the ground, reducing mechanical and thermal turbulent heat fluxes back to the boundary layer, cooling near‐surface air, and stabilizing the boundary layer. During the night, aerosols decreased boundary‐layer heating rates but increased downward infrared irradiances to the ground. Warmer ground temperatures increased mechanical turbulent heat fluxes to the boundary layer, increasing nighttime near‐surface temperatures. Thus, aerosols affected temperatures primarily through ground‐atmosphere turbulent heat transfer.